The present invention relates to an alignment cross hair apparatus used in connection with aligning a laser beam emitted from a laser.
Infrared lasers, such as yttrium-aluminum-garnet (YAG) lasers are used for scribing identification markings on semiconductor wafers. These identification markings can be located anywhere on the surface of the wafer and in any pattern desired. In manufacturing satisfactory identification markings, the power intensity, location and focus of the laser beam must be carefully controlled. To scribe silicon wafers, the laser beam must pass through several mirrors and lenses before reaching the marking surface, as shown by a typical layout illustrated in FIG. 1. Since the current art for making semiconductor wafers calls for marking tolerances within micrometers, laser beam placement and focus are crucial.
One of the problems inherent with scribing wafers is properly aligning the laser beam through the various optics. Ideally, the laser beam should be aligned so that the center of the laser beam strikes or passes through the center of the various optics along its path. Keeping the center of the laser beam in the center of the optics reduces distortion of the laser beam and minimizes power loss. However, the beam produced by a YAG laser falls in the infrared light spectrum, and cannot be seen by the naked eye. Therefore, to align the laser beam through the optics, either a phosphor-covered disk or an infrared viewing scope may be used to track the location of the laser beam. The phosphor-covered disk, when held in the direct path of the laser beam, produces a visible green image where the beam strikes the phosphor. An infrared viewing scope (a camera-like device) produces an image on a monitor displaying where the beam contacts a surface such as a mirror or lens.
Unfortunately, neither of these methods for viewing the laser beam produces a sharp image of the laser beam. The image produced by both the phosphor and the infrared viewing scope is not proportionally responsive to the intensity of the beam, but instead each has varying response thresholds and shows little differentiation between varying intensities of the beam. The accuracy of the laser beam placement is thus very subjective depending on the response curve and sensitivity of the infrared viewing scope and the phosphor block.
In addition, the properties of an infrared laser beam are such that viewing the beam position is always difficult and subjective. Each person responds differently to the image of the laser beam. This may be due to variations among individuals with respect to the sensitivity of the eye's rods and cones, the brain's interpretation of the image, ambient lighting conditions, and the eye's response to monochromatic light. Along with the eye-related difficulties are the actual properties of the laser beam. A typical laser beam is a circular, gaussian distribution of intensity that is not usually well centered, but instead has the point of greatest intensity displaced somewhat from the exact center of the beam. Thus, the position of greatest intensity of the laser beam may not coincide with the position of the center of the beam.
Adding to this difficulty, the mirrors and lenses used to position and focus the laser beam may be angled and rotated about each of the x, y and z axes to allow the laser beam to be aimed in any direction. However, when the beam strikes a mirror or lens at an oblique angle, this further blurs the image of the infrared laser beam as shown by the phosphor disk or infrared viewing scope, producing an oblong, distorted beam pattern with blurry and indistinguishable edges. Impurities in the optics will also distort the image of the laser beam. This further increases the difficulty of locating the center of the laser beam.
Accordingly, a need has developed that allows a laser beam operator to distinguish the center of the laser beam using different methods of viewing the laser beam, which does not depend upon the intensity of the laser beam, which allows alignment on oblique surfaces, which is not affected by impurities in the optics, and which does not distort the laser beam.